In recent years there has been an increased concern in the United States and elsewhere about air pollution from industrial emissions of noxious oxides of nitrogen, sulfur and carbon. Government agencies, in response to such concerns, have in some cases already placed limits on allowable emissions of one or more of the pollutants, and the trend is clearly in the direction of increasingly stringent restrictions. Petroleum fuel refineries are particularly affected by present and anticipated restrictions on emissions, particularly emissions of nitrogen oxides and carbon monoxide. Catalytic cracking, a major petroleum refinery process, is the largest single source of nitrogen oxides in such refineries.
Fluid catalytic cracking of petroleum hydrocarbons to lower molecular weight products is a well known process. This process is practiced industrially in a cyclic mode wherein hydrocarbon feedstock is contacted with hot, active, solid particulate catalyst and without added hydrogen at rather low pressures of up to about 50 psig and temperatures sufficient to support the desired cracking. As the hydrocarbon feed is cracked to more valuable and desirable products, "coke" is deposited on the catalyst particles. It should be noted that substantially all of the organic nitrogen in the feedstock to the cracker is incorporated with the coke deposit. The coked catalyst particles are disengaged from the hydrocarbon products and contacted with an oxygen containing gas in a regenerator whereupon coke is burned away from the particles to regenerate their catalytic activity. The heated, regenerated catalyst particles are then returned to the cracking zone and contacted with additional hydrocarbon feed and the cycle is repeated. The phrase "circulating inventory of cracking catalyst" as used herein includes the total catalyst contained in the cracking and regenerator sections of the cracking plant, including transfer lines, standpipe, etc.
In general, when coke is burned with a deficiency of oxygen, the regenerator flue gas has a high CO/CO.sub.2 ratio and a low level of nitrogen oxides, but when burned with excess oxygen, the flue gas has a high level of such nitrogen oxides and a reduced CO content. Thus, catalytic cracking regenerators emit CO, or nitrogen oxides, or mixtures of these pollutants with the flue gas in varying quantities, depending on feed quality, mode of operation of the regenerator, etc.
Although several nitrogen oxides are known which are relatively stable at ambient conditions, it is generally recognized that two of these, viz. nitric oxide (NO) and nitrogen dioxide (NO.sub.2), are the principal contributors to smog and other undesirable environmental effects when they are discharged into the atmosphere. These effects will not be discussed further here since they are well recognized.
Nitric oxide and nitrogen dioxide, under appropriate conditions, are interconvertible according to the equation EQU 2NO+O.sub.2 =2NO.sub.2.
For purposes of the present invention, NO.sub.x will be used herein to represent nitric oxide, nitrogen dioxide (the principal noxious oxides of nitrogen), and/or mixtures thereof.
U.S. Pat. No. 3,900,554 to Lyon describes a homogeneous gas phase thermal reaction to remove NO.sub.x from combustion effluent by adding 0.4 to 10 moles (preferably 0.5 to 1.5 moles) of ammonia followed by heating to 1600.degree. C. to 2000.degree. C. The NO.sub.x content is lowered as a result of its being reduced to nitrogen by reaction with ammonia. The so-called "selective catalytic reduction" type process which operates at a much lower temperature, 200.degree. to 600.degree. C., is exemplified by U.S. Pat. No. 4,220,632 to Pence et al., which describes a process for reducing NO.sub.x from a fossil fuel fired power generation plant, or from other industrial plant off-gas stream, to elemental nitrogen and/or innocuous nitrogen oxides by employing ammonia as reductant and, as catalyst, the hydrogen or sodium form of a zeolite having pore openings of about 3 to 10 Angstroms.
When incomplete combustion of coke in the regenerator of a fluid catalytic cracking plant leaves a significant amount of carbon monoxide (CO) in the flue gas, such flue gas not only emits CO to the atmosphere, but also tends to sporadically burn (by reaction of CO with residual oxygen) in ducts and flues of the plant (afterburning), damaging these structures by excessive temperatures. It is now known that trace amounts of a platinum group metal, such as 1.0 parts per million of platinum incorporated with the cracking catalyst, effectively catalyzes the complete burning of carbon monoxide to carbon dioxide in the regenerator without detriment to the cracking reaction. This development simultaneously eliminated the environmental problem and the problem of sporadic afterburning, and has been very widely accepted by refiners. Such catalysts and their use are described in U.S. Pat. Nos. 4,251,395, 4,265,787, 4,088,568, 4,072,600, 4,093,535 and 4,159,239, all to Schwartz, and are incorporated herein by reference for further details on composition and use. As described therein, such promoted catalysts may be used to completely burn CO (referred to hereinbelow as "full CO-combustion") or to only partially burn the CO (referred to hereinbelow as "partial CO-combustion") by the simple expedient of limiting the oxygen supplied to the regenerator. The term "Pt group metal CO-oxidation promoter" as used herein means those metals and their mode of use as taught by the Schwartz patents cited above. Operation with CO-combustion promoters, which solves the CO emissions problem, can in some instances increase the NO.sub.x emissions.
Although most petroleum cracking is conducted with fluid beds, non-fluid catalyst beds also may be used. Such processes also are operated continuously and in cyclic fashion, with movement of the catalyst against gravity being effected, as needed, pneumatically or mechanically. One typical example of industrially practiced moving bed hydrocarbon catalytic cracking is known as Thermofor Catalytic Cracking (TCC). In this process the catalyst is in the shape of beads or pellets having an average particle size of about 1/64 to 1/4 inch, preferably about 1/8 inch. In contrast with these particles, in fluid catalytic cracking (FCC) the catalyst is a fine powder of about 10 to 200 microns, preferably about 70 micron size. The problems encountered with NO.sub.x and CO emissions in fluid catalytic cracking exist also with the moving bed type processes. Although the description herein, for simplicity, is couched in terms of fluid catalytic cracking, it is contemplated that the invention encompasses both fluid and moving-bed type processes.
The catalysts used in endothermic nonhydrogenative cracking are to be distinguished from catalysts used in exothermic hydrocracking. Operating conditions also are different. While the catalytic cracking processes to which this invention is directed operate at low pressures near atmospheric and in the absence of added hydrogen, hydrocracking is operated with added hydrogen at pressures of up to about 1000 to 3000 psig. Further, nonhydrogenative catalytic cracking is a reflexive process with catalyst cycling between cracking and regeneration (coke burn off) over a very short period of time, such as seconds or minutes. In hydrocracking, on the other hand, a fixed bed of catalyst usually remains in cracking service for months between regeneration. Another important difference is in the product. Nonhydrogenative catalytic cracking produces a highly unsaturated product with substantial quantities of olefins and aromatics, and a high octane gasoline fraction. Hydrocracking, in contrast, produces an essentially olefin-free product and a relatively low octane gasoline. The present invention is not directed to hydrocracking nor is it within the scope of this invention to use hydrocracking catalysts.
We have now found that NO.sub.x emissions formed in cracking catalyst regeneration can be dramatically reduced or eliminated without resorting to the very high temperatures required for the gas phase thermal reaction with NH.sub.3, and without adding ammonia at all, and without providing expensive downstream facilities to clean up the flue gas.
It is an object of this invention to provide a novel cracking catalyst composition which reduces emissions of NO.sub.x and/or CO from the regenerator of a fluid catalytic cracking plant.
It is a further object to provide an improved fluid catalytic cracking process that utilizes such catalyst.
It is a further object of this invention to provide a means for reducing NO.sub.x emissions when operating a fluid catalytic cracking plant in the full CO-burning mode.
These and other objects of this invention will become apparent from this entire specification including the appended claims.